1. Field of the Invention
The present invention relates to a method of fabricating a polysilicon film, and more particularly, to a method of fabricating a polysilicon film by an excimer laser crystallization(ELC) process.
2. Description of the Prior Art
The progress of science and technology has led to small, effective, and portable intelligent information products becoming a part of our lives. Display devices play an important role because all intelligent information products, such as mobile phones, personal digital assistants (PDAs), or notebooks, need display devices to be a communication interface. However, the fully developed amorphous silicon thin film transistor liquid crystal display (a-Si TFT LCD) devices, which are restricted in their carrier transfer rate, have difficulty in meeting the requirements of being thin, being power saving, and having high resolution. Therefore, the a-Si TFT LCD devices are gradually replaced by low temperature polysilicon (LTPS) thin film transistor liquid crystal display (TFT LCD) devices.
In the liquid crystal display devices, since a normal glass substrate can only work at a temperature below 600° C., fabricating a polysilicon film directly under a high temperature will make the glass substrate twisted. Thus, in a conventional method of fabricating a polysilicon thin film transistor, an expensive quartz substrate is needed and only a small size liquid crystal display panel can be made. Recently, a method of forming a low temperature polysilicon film by crystallizing an amorphous silicon film is gradually developed. Among the methods of forming polysilicon film, the excimer laser crystallization process is the major focus.
The excimer laser crystallization process is typically divided into two types, which are a conventional scanning ELC process and an ELC process with a location-controlled grain boundary. Please refer to FIG. 1, which is a schematic diagram of a conventional scanning ELC process. As shown in FIG. 1, a buffer layer 12 and an amorphous silicon film 14 are deposited on a glass substrate 10 in sequence. Then, the glass substrate 10 is put on a carrying stage, which moves step by step along an X direction. In addition, the excimer laser L scans the whole surface of the amorphous silicon film 14 step by step and heats the amorphous silicon film 14 rapidly, thereby making the amorphous silicon film 14 melted and crystallize to a polysilicon film 16 thereafter. Though this method is very simple, the location of the grain boundary cannot be controlled so as to restrict the transfer rate of electronics and the performance of devices.
Please refer to FIG. 2 and FIG. 3, which are schematic diagrams of an ELC process with a location-controlled grain boundary. As shown in FIG. 2 and FIG. 3, in this method, a buffer layer 22 and an amorphous silicon film 24 are also formed on a substrate 20 at first. The amorphous silicon film 24 is defined with a first region 26 and a second region 28 in the surface of the amorphous silicon film 24. Then, a patterned mask layer 30 is formed to cover on the second region 28. Normally, the mask layer 30 comprises a metal layer, which can increase the reflection rate to reduce the heat absorption of the amorphous silicon film 24, or a silicon nitride layer, which can increase the thermal conductivity to increase the heat dissipation of the amorphous silicon film 24. As a result, this makes the amorphous silicon 24 in the second region 28, which is covered with a mask layer 30, partially melted and the amorphous silicon film 24, which is not covered with the mask layer 30, fully melted. Consequently, when the melted amorphous silicon film 24 solidifies after the excimer laser irradiation is finished, a nucleation will happen based on the partially melted region since there is a heterogeneous interface between the partially melted region and the completely melted region. Grains are grown laterally from the second region 28, which is partially melted, to the first region 26, which is completely melted, to form a polysilicon film in the first region 26. Then, a photo-etching process is performed to remove the mask layer 30 and the amorphous silicon film 24 in the second region and form a polysilicon island 32 in the first region 26. After that, latter processes of liquid crystal panel manufacturing are performed while the polysilicon island 32 is used as an active area to form a driving circuit of the liquid crystal display panel.
In the aforementioned excimer laser crystallization process with a location-controlled grain boundary, though the location of the grain boundary can be controlled, the produced grain size is only about 3 μm due to a limitation of the room temperature. In addition, the use the metal layer on a conductor directly easily pollutes the active area of devices, and the use of the silicon nitride layer often makes a phenomenon of semiconductor film peeled due to the high concentration of hydrogen atoms contained therein. Thus, there is a strong need to develop a fabricating method of the polysilicon film to increase the grain size in the polysilicon film.